What to look for in a paintball marker (gun)

What to look for
Not all paintball guns have the same value for the price.  Just about every marker under $400 has junk stock parts, and needs some upgrade to get good results.

Accuracy
Accuracy is determined by several things. 80% of accuracy is determined by the barrel. 15% is due to the regulator; if the air rises or falls in pressure, the ball will go further or less far, and hit higher of lower. A good air regulator will maintain constant pressure and therefore accuracy. 5% of accuracy is due to the bolt system and operating pressure. Thus, the barrel is the first thing you should upgrade. Whatever your equipment, you must have a good paintball to barrel match.

Bolt
Made from metal or delrin (a strong and light plastic).  Both are good.

Barrel
Stock barrels typically are pretty poor.  Aftermarket barrels are more accurate, are quieter due to porting, have better honing (the finish of the bore is smoother resulting in less friction), or look better.

Paintball to barrel match
To prevent ball breaks in the barrel and maximize accuracy requires that the paint match the barrel. Put a ball in the barrel and blow it out.  If it rolls out (too small) or gets stuck (too big), you have a bad match.

Paintballs expand when warm or humid, and contract when cold or dry.  You may want to have three barrels for small, medium, and large paint.  That way you can match the paint to the bore while on the field. Often, the field will require you to buy their paint. The Freak barrel system and the Empire 4-piece give you a barrel with inserts of different diameters to match the paint you are using. And the Psycho Ballistics Aradus can adjust the barrel bore size with the twist of the barrel. But these systems may cost more than the gun itself if you buy a cheap gun.

Barrel length
You will do fine with a barrel between 10" and 14".  Shorter is less accurate and louder; longer has more friction and uses more gas to get to speed, is clumsy in the field, harder to snap shoot, and easier to see popping out from behind a bunker.

Low pressure (less than about 200PSI) guns may require a longer 12-14" barrel to reach speed.  High pressure guns (about 800PSI) may only need 10-12".  If you use low pressure and a really short 6-8" barrel, no porting is mandatory or you won't get up to speed, or use too much gas; and count on less range and accuracy.  Longer barrels do not increase accuracy, but they are quieter.

Porting
Holes placed in the barrel make the gun quieter, and reduce turbulence making it more accurate. Too many port holes can make the barrel less efficient (a gas hog); you will get fewer shots per tank of CO2 or HPA. Port holes also make it harder to clean the barrel since paint gets stuck in the holes that a squeegee can't get.

Types of markers
Electro-pneumatic
The firing system is all controlled electronically. This allows for firing of the marker with less effort than it requires to click a button on your mouse. It also enables markers to have several different firing modes such as 3 shot bursts, 6 shot bursts or even fully automatic. However, virtually all tournaments and paintball fields only allow semi-automatic mode (1 trigger pull, one shot). Because of this, some high end markers ship with a control board only allowing semi-automatic, and for fully auto modes the board will need to be replaced. Others rely on LCD or LED indicators to indicate that a non-semiautomatic mode has been selected; some guns have a jumper that you can remove to lock the gun into semi-auto mode when necessary.

Mechanical
The action is controlled solely through mechanical means. Many mechanical markers have a hammer which when cocked is held back by a catch connected to the trigger. It will also have a spring trying to push the hammer forward. When the trigger is pulled, the catch is released and the hammer is allowed to slam in to the valve. This diverts the flow of air from the tank, through the bolt and into the paintball, propelling it out the barrel. Excess air not used to propel the ball is then used to re-cock the hammer. This type of marker is called a blow-back design and is the most common approach used. The Kingman Spyder line of markers are examples of blow back design.

Electro mechanical
A hybrid approach, where the mechanical firing of the marker is actuated via an electric coil. This allows for the short light trigger associated with electronic markers on an otherwise mechanical marker. The Kingman markers using their ESP trigger, and the E-Mag by Airgun Design, are examples.

There are many markers brands including Air Gun Designs, Tippmann, Kingman, Worr Games Products, JT, Sheridan, WDP, etc.

Air
Because CO2 becomes a liquid when compressed, it needs to expand to a gas to be used by the paintball gun. This expansion causes the tank to cool as the liquid CO2 turns into gas. Eventually, under sustained fire, and especially in cold weather, the tank can become so cold that the liquid CO2 will not evaporate into gas, and liquid CO2 will enter the gun causing the gun to freeze. This can cause damage to internal seals and will also put the gun out of commission for a good 15 minutes while it warms back up.

When nitrogen is compressed, it remains a gas. When it expands, it also cools the tank, but at an unnoticeable rate because it does not have to transition from liquid to gas. Therefore it is viewed as a superior source of propulsion. However, because HPA (High Pressure Air) is stored at up to 4,500 lb/in² while liquid CO2 is stored at 1,200 lb/in², tanks for HPA are more expensive. Nitrogen tanks can ether be filled with pure N2 or compressed air, which is 79% N2.

Nitrogen and air systems are more expensive, and are preferred to CO2. These air sources are primarily used by people who play often and have tournament-grade markers.

Quality vs. Price
The higher end guns that actually shoot 20 balls per second (BPS) and above, never chop balls, have excellent accuracy and gas efficiency, cost anywhere from $800 to $1500.

No low-end blow-back marker can cycle itself faster than about 13 BPS without shaking itself to pieces. The electronic circuitry to cycle at 20 BPS is the easy part, but you have a heavy metal hammer hitting back and forth against your valve and springs, and if you are running at 700-800 PSI, a lot of force is being produced and no marker can survive that kind of punishment. Even the expensive Angels and Intimidators are made to operate at pressures below 150 PSI, and they have no internal springs.

Some of the gas that propels the ball forward is escaping into the feed port and up the hopper, pushing the incoming balls back out. If it delays the next paintball from falling into the breach, this will cause dry firing or chopping. High end markers that never chop balls, have a sensor ("eye") that will not allow the gun to fire until the paintball is in its place.

Honing, or the finishing/polishing process of the barrel, determines the amount of friction.  More friction means the ball moves slower, and that may lead to more paintball breaks in the barrel.